This invention relates generally to systems for converting thermal and radiant energy into electrical energy, and more particularly the invention relates to thermophotovoltaic conversion systems and to photovoltaic cells useful therein.
Solar radiation is recognized as an unlimited source of energy if properly harnessed. Many techniques have been suggested for converting solar radiation into thermal energy, and in recent years much attention has been directed to the direct conversion of solar radiation into electrical energy through use of photovoltaic cells.
The photovoltaic cell comprises a substrate of semiconductive material having a P-N junction defined therein. In the planar silicon cell the P-N junction is formed near a surface of the substrate which receives impinging radiation. Radiated photons create mobile carriers (holes and electrons) in the substrate which can be directed to an electrical circuit outside of the cell. Only photons having at least a minimum energy level (e.g. 1.1 electron volts for silicon) can generate an electronhole pair in the semiconductor material. Photons having less energy are either not absorbed or are absorbed as heat, and the excess energy of photons having more than 1.1 electron volts energy (e.g. photons having a wavelength of 1.1 microns and less) creates heat. These and other losses limit the efficiency of photovoltaic cells in directly converting solar radiation to electricity to the order of 10-20%.
Attempts at increasing the efficiency of photovoltaic cells have focused on the actual cell structure and on limiting wavelengths, and thus photon energy, of impinging radiation. One technique for achieving a bandemission spectrum of radiation is thermovoltaic conversion as described by Wedlock in "Thermo-Photo-Voltaic Energy Conversion," Proceedings of the IEEE, May, 1963, pages 694-698. As described by Wedlock, thermal radiation is absorbed by a black body radiator which, in turn, generates a spectrum of radiation containing photons with energy levels in excess of a threshold required to generate the carriers in a photovoltaic cell. The photovoltaic cell is illuminated with the radiated spectrum from the black body radiator. U.S. Pat. No. 3,929,510 to Kittl discloses a solar radiation conversion system employing thermo-photovoltaic conversion.
Improvements in cell structure have focused primarily on increased absorption of photons and increased lifetime of minority carriers created by the absorption of photons. Kim and Schwartz, IEEE Trans. on Electron Devices, ED-16, No. 7, July, 1969, pages 657-663 popose an intrinsic photovaltaic diode. The significant factors necessary for the proposed P-I-N structure to be effective include high doping of the N and P regions, a high lifetime intrinsic region, and surface treatment to minimize surface recombination of holes and electrons and to minimize reflection loss. The cell proposed by Kim and Schwartz comprises a substrate of intrinsic germanium having spaced alloy regions of alternating conductivity type in one surface of the substrate. Kim and Schwartz recognize the physical limitations of such a structure for silicon photo cells.